A laboratory study of graupel charging

PEREYRA, RODOLFO GUILLERMO; AVILA, ELDO EDGARDO; CASTELLANO, NESVIT; SAUNDERS, CLIVE

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2000 vol. 105 p. 20803 - 20812

0148-0227

Measurementshavebeenmadeof chargetransferwhenvaporgrownice crystals , rebound from a riming target representing a graupel pellet falling in a thunderstorm. Earlier studies in the laboratory in Córdoba of charge transfer between an individual falling ice sphere and a riming target noted that the sign of the charge lransfer was dependent upon temperature and effective liquid water content (EW). The new work uses a similar experimental technique; however, a cloud of ice crystals is grown in order to study multiple interactions with the riming target The résults also show charge sign dependence on temperature and EW; positive rimer charging is observed at high temperatures and for low and high values of EW at low temperature, while negative rimer charging is noted at low temperatures for intermediate values ofEW, These results are similar to those obtained by Takahashi (1978) and, as has been reported before, are ratherdifferent from those obtained in Manchester by Jayaratne et al. (1983), Saunders et al. (1991), and Saunders and Peck (1998), Significant differencesbetween the two types of data sets' are attributed to the experimental techniques used in the various studies. In the present work the ice crystal cloud and the cloud of supercooled droplets responsible for riming the target are grown in separate chambers and then mixed shortly before the crystals and droplets encounter the riming target, so that the droplet cloud is not depleted by the growing ice crystals. In the Manchester experiments, the ice crystals grow in the same supercooled droplet cloud used to rime the target It is possible that the mixing process provides an undepleted droplet cloud and afl transient enhanced vapor supply that affects both the ice crystal and graupel vapor depositional growth rates, leading to the present results.